Direct fuel injection (DI) systems provide some advantages over port fuel injection systems. For example, direct fuel injection systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. However, direct fuel injectors may not be able to provide a desired amount of fuel to a cylinder at higher engine speeds and loads because the amount of time a cylinder stroke takes is shortened so that there may not be sufficient time to inject a desired amount of fuel. Consequently, the engine may develop less power than is desired at higher engine speeds and loads. In addition, direct injection systems may be more prone to particulate matter emissions.
In an effort to reduce the particulate matter emissions and fuel dilution in oil, very high pressure direct injection systems have been developed. For example, while nominal direct injection maximum pressures are in the range of 150 bar, the higher pressure DI systems may operate in the range of 250-800 bar.
One issue with such high pressure DI systems is that when the engine is configured with both direct fuel injection and port fuel injection (DI-PFI systems), the system is limited to operating the port fuel injection system at low pressure conditions. In other words, high pressure port fuel injection, such as higher than 5 bar, may not be possible without the inclusion of an additional dedicated pump. As such, while there may be conditions when high pressure port fuel injection is desirable, the addition of another pump for raising the pressure of the port injection system may add cost and complexity. Another issue with such high pressure DI systems is that the dynamic range of the injectors may be limited by the rail pressure. Specifically, when the rail pressure is very high and the engine has to operate at low loads, the direct injector pulse width may be very small. Under such small pulse width conditions, direct injector operation may be highly variable. In addition, at very low pulse widths, the direct injector may not even open. These conditions can result in large fueling errors.
In one example, the above issue may be at least partly addressed by a method for an engine, comprising: operating a high pressure fuel pump to deliver fuel at a variable pressure to a first fuel rail coupled to direct fuel injectors, and at a fixed pressure to a second fuel rail coupled to port fuel injectors, the fuel delivery controlled via a mechanical spill valve of the pump, wherein the second rail is coupled to an inlet while the first rail is coupled to an outlet of the pump. In this way, the specific configuration of the fuel rails relative to the high pressure fuel pump, as well the use of a mechanical spill valve and various additional check valves, enables a single high pressure fuel pump to be used to provide a substantially higher port fuel injection pressure.
As an example, a fuel system may be configured with a low pressure lift pump and a high pressure injection pump. The high pressure pump may be a piston pump. An output of the high pressure injection pump may be controlled mechanically, and not electronically, via the use of a magnetic solenoid valve (MSV). At least one check valve and one pressure relief valve (or over-pressure valve) may be coupled between the lift pump and the injection pump. A first fuel rail delivering fuel to direct fuel injectors may be coupled to an outlet of the injection pump via a check valve and a pressure relieve valve. Likewise, a second fuel rail delivering fuel to port fuel injectors may be coupled to an inlet of the injection pump, also via a check valve and a pressure relieve valve. An unenergized MSV enables a fixed pressure of the second fuel rail to be raised substantially higher than the fuel pressure provided by the lift pump. For example, the pressure of the second fuel rail delivering fuel to port injectors can be raised to the same level as the minimum pressure of the first fuel rail delivering fuel to direct injectors (such as at 15 bar). The pressure of the first fuel rail may be further raised and varied by adjusting the pump output via the MSV. Thus, based on engine operating conditions, fuel may be delivered at high pressure to an engine cylinder via port injection and/or via direct injection. Further, during conditions when fuel delivery via high pressure direct injection is limited, such as during cold-starts (and extreme cold-starts) or when engine exhaust emissions are particulate matter limited, direct injection may be disabled and fuel may be delivered via one or more high pressure port injections.
In this way, port fuel injection may be provided at fuel pressures that are higher than the default pressure provided by a lift pump. More specifically, a high pressure displacement pump can be advantageously used for providing variable high pressure to a direct injection fuel rail while also providing a fixed high pressure to a port injection fuel rail. By raising the port injection default pressure to be as high as the direct injection minimum pressure, various benefits of high pressure port injection can be achieved. For example, fuel can be port injected at high pressure without incurring particulate matter issues associated with direct injection. In addition, smaller amounts/volumes of fuel can be port injected more accurately when direct injection of the equivalent amount is limited by the pulse-width or dynamic range of the direct fuel injector. Overall, fuel injection efficiency is increased and fueling errors are reduced, improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.